Return io.ErrClosedPipe from closed udpMuxedConn

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 50.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 79.21%. Comparing base (75ca3a2) to head (9485c82).

Files	Patch %	Lines
udp_muxed_conn.go	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #689 +/- ## ======================================= Coverage 79.21% 79.21% ======================================= Files 41 41 Lines 3733 3733 ======================================= Hits 2957 2957 Misses 547 547 Partials 229 229 ``` | [Flag](https://app.codecov.io/gh/pion/ice/pull/689/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | Coverage Δ | | |---|---|---| | [go](https://app.codecov.io/gh/pion/ice/pull/689/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `79.21% <50.00%> (ø)` | | | [wasm](https://app.codecov.io/gh/pion/ice/pull/689/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion) | `22.16% <0.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pion#carryforward-flags-in-the-pull-request-comment) to find out more.

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